College of Earth Sciences, Jilin University, Changchun 130061, China

This study presents LA–ICP–MS U–Pb dating and whole-rock geochemical analyses of the Late Triassic Gangshan harzburgite in the Qingyuan area, with the aim of elucidating its petrogenesis and further constraining the early Mesozoic tectonic evolution of the eastern segment of the northern margin of the North China Block (NCB). Zircons from the harzburgites exhibit typical oscillatory growth zoning or striped absorp tion in cathodoluminescence images. U–Pb analyses of zircons yield ages ranging from 2 525 Ma to 225 Ma, with two youngest ages (225±7 Ma) indicating that the harzburgites were formed during Late Triassic. Geochemical analyses of the Gangshan harzburgites show that the rocks have low concentrations of SiO2 (42.38%–42.85%) and Al2 O3 (3.31%–3.33%), along with high concentrations of MgO (41.32%–41.76%), Cr (4 856×10-6–5 191×10-6) and Ni (1 942×10-6–2 041×10-6). They also display low REE abundances (∑REE=4.38×10-6–4.69×10-6) and flat REE patterns with low (La/Yb)N ratios (1.24–1.56) and slightly po- sitive Eu anomalies (δEu=1.13–1.16). These features suggest that the Gangshan harzburgites are cumulates of basaltic magma derived from the depleted lithospheric mantle. Combined with previous studies, these Late Triassic mafic–ultramafic rocks, together with coeval granitoids in adjacent regions, constitute a typical bimodal association, suggesting that they formed in a post-orogenic extensional environment after the final closure of the Paleo-Asian Ocean.